This invention relates generally to fuses, and, more particularly, to time delay fuses.
Fuses are widely used as overcurrent protection devices to prevent costly damage to electrical circuits. Fuse terminals typically form an electrical connection between an electrical power source and an electrical component or a combination of components arranged in an electrical circuit. One or more fusible links or elements, or a fuse element assembly, is connected between the fuse terminals, so that when electrical current through the fuse exceeds a predetermined limit, the fusible elements melt and opens one or more circuits through the fuses to prevent electrical component damage.
A time delay fuse is a type of fuse that has a built-in delay that allows temporary and harmless inrush currents to pass through the fuse without opening the fuse link or fuse links, yet is designed to open upon sustained overloads or short circuit conditions. For example, conventional time-delay fuses typically allow five times the rated current for up to ten seconds without opening, and therefore are particularly suited for applications including circuits subject to inrush current transients, such as electric motors that draw relatively large motor starting currents of a relatively short duration as the motors are energized. In certain circumstances, however, it is desirable to provide a longer time delay than is typically possible with conventional time delay fuses.
In an exemplary embodiment of the invention, a fuse element for a time delay fuse includes a conductive fuse element member, a fuse link formed within the member, and a heat sink coupled to the member. The heat sink draws heat from the fuse element member and prevents the fuse link from opening for an increased amount of time during relatively high current overload conditions, while substantially unaffecting time delay performance at relatively low current overload conditions.
More specifically, the heat sink is a nickel thermal load in one embodiment of the invention. The fuse element member is substantially flat and includes opposite faces, and the heat sink is coupled to and engages the opposite faces to ensure heat transfer from the fuse element member. In a further embodiment, the heat sink is U-shaped and wraps around the fuse element member.
The heat sink may be used in combination with other known time delay features for improved effectiveness. For instance, in one embodiment, the conductive fuse element member includes an outer surface and is fabricated from a first conductive material, and the fuse element includes a low melting alloy fabricated from a second material applied to the outer surface. This results in a known M effect, wherein the fuse element operates at lower temperatures than it would otherwise operate in the absence of the low melting point alloy. Combined effects of an M effect alloy and the heat sink substantially increase time delay performance of the fuse element at relatively high overload currents, thereby preventing premature opening of the fuse element during relatively high transient overload currents.